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My guess is it is from 宣同学, "同学" means classmate, basically it is a title that you can add to anyone's name. E.g. if I want to mention your id, not too formally, then I say "goonsack 同学". And 宣 means propaganda, as in the word 中宣部.

Classmate Xuan is a new nick name, these words are changing constantly though to try and outlast the giant machine of censorship in China. New nicknames are created quite often for various things such as ZF for government, or "Zheng Fu". Grass mud horse is another popular one. Chinese netizens are quite clever at finding ways to say censored words on the internet, using different characters that sound the same for instance. "Classmate Xuan" will probably be censored later and it will change to something else.

Strange times. I think that there is a chance of a new online Tiananmen. Once again the vast majority of the country will be left relatively clueless while the educated minority fight in vain for their freedoms.